is there a cheap way to use a 120ac to 20vdc power brick to power 12v devices?
A brick is up to 90watt, so is there a trick to do that for a few bucks soldering the
right parts... either 20vdc to 12vdc, or lowering the ac input from 120ac to whatever
so the laptop brick outputs 12v?
thanks!
For low power (maybe a couple-hundred milliamps) you can use an LM7812 (and a couple of capacitors). The LMxx regulator is rated for 1 Amp, but only with a heatsink and probably not with 8V "dropped" across it.
For higher power, you'll need a switching regulator which requires a couple more parts including an inductor. 12V at 90W is 7.5 Amps and off the top of my head I don't know of a switching regulator that can handle that much current. So, you'll have to search yourself.
It's kind-of redundant to add a switching regulator after a switching power supply... The best approach is probably to just buy the 12V power supply you want.
or lowering the ac input from 120ac to whateverso the laptop brick outputs 12v?
That won't work with a regulated power supply, since it's trying to hold the 20V regardless of input voltage variations or output load variations. A 90W laptop supply typically is well regulated.
At some point the output voltage will drop but it won't drop in a predictable way and it might get (electrically) noisy when operating out-of-spec. Plus, lowering the 120VAC input power is not easy... That's not how light dimmers work.
Use a step-down dc-dc converter
Or just get a 5 volt supply to begin with
Either way pretty cheap. You can't beat the chinese ebay sellers for generic, basic stuff like that.
Indeed, the 7812 or 7805 is great if you draw like no current (powering an Arduino, maybe some leds, nothing fancy). If you need more ampssss go with a switching step down converter. The full 90W is a bit hard but 2A (okay, most 3A modules are great up untill 2A :p) is already 24W. So it really depends on what you want to power.
If you can open up the PSU you can probably modify it to trick it to output less voltage. But there is no universal way and you have to know what you're doing.
s there a cheap way to use a 120ac to 20vdc power brick to power 12v devices?
You might use a UBEC voltage controller similar to those below.
or select from range:
or smaller
you can get a 1amp switching regulator that drops to 5v from adafruit:
better heat than a linear regulator, imo...
I've been suggesting the Murata Oki for several years now. I think LarryD might have suggested them originally.
I've been suggesting the Murata Oki for several years now. I think LarryD might have suggested them originally.
Yeah, I bought one after reading one of your suggestions. (I forgot that was a dc-dc converter) (haven't used it for awhile now but it works great)
raschemmel:
That's a great idea. A drop-in SWITCHMODE replacement for a 7805 !it's about time.
The recom link from lemming is a direct 7805 drop in and is way cheaper than the one from adafruit. There is a 1a version also.
The oki one looks interesting. I'll probably try it next time I need something over 1a.
I'm looking for a converter that can take 48v or higher input. meanwell has them but a bit expensive. I'm using LM2956HV based modules for now.
While I agree with all of the suggestions to use switching regulators and the like - linear regulators of the 78xx series can be configured to allow output of more than an amp or two.
Besides there being variants that support larger currents (mainly in TO-3 metal cans) - you can add a current pass transistor (typically a 3055 NPN variant) to the system; googling "78xx pass transistor" should get some results - in fact, I think such a circuit is covered in the datasheets for the regulators.
In either case, both devices usually need to be heatsinked (generally on something really hefty) to handle the loads - but it can be done, if size, weight, and efficiency aren't important.
I built a DIY 15A 5V regulated power supply using an LM7805 and a 2n3055 as one of my first projects when I started in electronics in 1979.
is there a cheap way to use a 120ac to 20vdc power brick to power 12v devices?
How much 12V current do you need?
12V, 10A, $12
I've used this one to drive 70+ 12V LED strings, pushing out 7+ amps.
Check out some of the other adjustable regulators also:
http://www.dipmicro.com/store/index.php?act=viewCat&catId=690
Those are some very good prices.
Thanks guys, lots of alternatives indeed, many infos I can use beyond the current need.
DipMicro seems to have great prices, but must factor in shipping which is 50%-120% of the price. Their 5V/2A power plugs are like $4.
I think any converter that's past the price point of a new power supply (~$10 to $15) is the wrong route. I think I'll look at chinese buck converters for smaller 2A power needs (I'm thinking of putting 12V black lights on a bike, using a usb power bank, while driving other stuff with it so it's a good alternative). I could use multiple cheap buck converters if big ones get very pricey.
A friend told me chinese converters tend to fail after a short while and burst in flames so I would save that for low reliability needs. 
Dipmicro has very inexpensive shipping to the US generally. I just ordered $90 worth of stuff, shipping was $6.80.
cr0sh:
you can add a current pass transistor (typically a 3055 NPN variant) to the system;
it's a variant all right!
More specifically, a 2N2955 PNP.
That reminds me of my first project when I started in electronics in 1979. I built a supply like that into a wooden cigar box using ring terminals and machine screws from the inside out with two nuts and washers on the outside as terminals. It was very low tech but worked great. I had a TO-3 heatsink.
Paul_B thanks!
Electronics are such faint memories for me nowadays.
The great thing in this case the brick's voltage is irrelevant if within specs.
So it seems a 7812 with a transistor and 2 resistors would suffice. Any sizing of R needed? I suppose even 1/4W would be OK for 10R since they feed the base of the transistor, but the 1R is likely 1 or 2 watt?
http://www.reuk.co.uk/High-Current-Voltage-Regulation.htm
I suppose the caps may not really needed either since the brick's voltage is regulated.